Scientists conduct a targeted search for alien life in the Solar System and beyond since the second half of the 20th century. According to current scientific understanding, the probability of detection of highly organized life on all planets of the Solar System is very low. Studies of meteorites, the upper layers of the Earth’s atmosphere, and the data collected by the Viking Space Program allow some scientists to argue that the simplest forms of life could exist on some objects of the Solar System. Astrobiologists continue their search for such simple life forms as bacteria or protozoa. The gas giants’ moons are considered to be promising for finding the extraterrestrial life in the Solar System.
Titan is the largest moon of Saturn and the second largest moon in the Solar System. The Titan’s atmosphere is dense, chemically active, and rich in organic compounds. Its atmosphere also contains hydrogen, ethane, and methane, which can be combined with some of the organic compounds (e.g. acetylene) for generating energy and development of life.
In June 2010, scientists reported the presence of anomalies in the Titan’s atmosphere after analysis of the data from the Cassini-Huygens mission. On this basis, they hypothesized the existence of primitive biological organisms there. According to this hypothesis, the organisms could absorb gaseous hydrogen and acetylene molecules, while the process of their life activity would form methane. As a result, a lack of acetylene and reduction of hydrogen near the surface would be observed on Titan (Cook & Weselby, 2010).
According to the data published by the end of June 2012, the ocean consisting of water with a small amount of salt should be under the surface of Titan (Cook & Brown, 2012). The new research published in 2014 suggested that the liquid in the ocean of the Saturn’s moon had high density and extreme salinity (Meltzer, 2015). Most likely, the liquid is the brine comprised of salts containing sodium, potassium, and sulfur. Furthermore, ocean depth varies in different parts of the satellite. The liquid is frozen in some places. Thus, it increases the inside ice crust that covers the ocean, so a layer of liquid in these areas practically does not reach the surface of Titan. Strong salinity of the subsurface ocean makes the existence of life in it virtually impossible.
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Enceladus is the sixth largest moon of Saturn. Enceladus, as well as Titan, causes a great interest among scientists. They began to surmise that Enceladus is geologically active since the era of Voyagers, which made the first photos of this small planet-satellite in the early 1980s (Meltzer, 2015). It has become quite clear that Enceladus is arranged much more complicated and fraught with more mysteries than it has been thought since the beginning of the Cassini-Huygens mission.
The surface of Enceladus is colder than that of other planets-satellites of Saturn; the average temperature is -200 °C. Such a low temperature is explained by the extremely high rate of albedo, which amounts to 90%. None of the known life forms can survive on the surface with such a low temperature. However, the surface of Enceladus has many fractures. They helped to establish the fact that there is an ocean under the ice crust. The temperature in the fractures can reach -85 °C, which is significantly higher than the surface temperature. Moreover, according to the Cassini, the depths of Enceladus have a hydrocarbon mix of liquid water and a source of heat, that is, all the key ingredients for the emergence of primitive life forms (Meltzer, 2015).
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There is the ocean of liquid water at a depth of 15-20 km at the South Pole under the ice crust of Enceladus. The temperature of the upper layers of the ocean is about -45 °C. However, the deeper immersion is, the higher the temperature is. It can reach up to about 0-1 °C, which is comparable to the temperature of water in some places on the Earth. In June 2011, the scientists using the Cassini found that the water in the ocean is salty and the composition is very close to that of the Earth (Meltzer, 2015). All of these discoveries have greatly increased the probability of life on Enceladus.
Europe is the sixth satellite of Jupiter. Nowadays, Europe is seen as one of the main places in the Solar System where the life may originate. Life could exist in the subsurface ocean or in the environment, which is likely to be similar to the terrestrial deep hydrothermal vents or the Antarctic Lake Vostok. Perhaps, this life resembles microbial life in the ocean depths of the Earth. Currently, there are no signs of the existence of life on Europa, but the likely presence of liquid water encourages scientists to send research expeditions for a closer study of the satellite.
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In 2009, Richard Greenberg calculated that the amount of oxygen in the ocean of Europe may be sufficient to support the development of life (Atkinson, 2009). Oxygen which arises from the decomposition of ice by cosmic rays can penetrate into the ocean with stirring of ice layers by geological processes, as well as through the fractures in the crust of the satellite. According to Greenberg, Europa’s ocean could achieve a greater concentration of oxygen through this process than the Earth’s oceans within a few million years (Atkinson, 2009). This would allow Europe to support not only the anaerobic microscopic life, but also such large aerobic organisms as fish. Given the low temperatures and high pressure on Europa, Greenberg suggests that the ocean of the satellite oxygenates much faster than that of the Earth. Also, he hypothesizes that the microorganisms could have gotten to the surface of the Jupiter’s moon along with meteorites (Atkinson, 2009).
In early April 2013, the scientists of California Institute of Technology reported that they had found large reserves of hydrogen peroxide on Europe (Carroll & Lopes, 2013). This is a potential energy source for the bacteria-extremophiles which theoretically can live in under-ice ocean of the satellite.